1. Field of the Invention
The present invention pertains to a method and apparatus for transferring heat from a substrate to a chuck that supports the substrate.
2. Discussion of the Background
There is a never-ending demand for increasing throughput in semiconductor, display and other types of substrate manufacturing. Many of the processes in substrate processing involve placing the substrate, such as a semiconductor wafer, on a chuck and processing the substrate. During certain of these processes, the substrate heats up, and this heat needs to be dissipated quickly. Quick heat dissipation allows the substrate temperature to be maintained within certain limits determined for the process even at high processing tool power levels, and the quick initiation of the next process step. Both of these allow a high process throughput, which drives down the process cost per substrate.
One process used with substrates, such as in the fabrication of semiconductor devices (e.g., integrated circuits, or “ICs”) or displays, involves subjecting the substrate to a plasma for depositing material onto or etching material from the substrate surface. During this process, high-energy plasma particles bombard the substrate, and generate a large amount of heat, which is absorbed by the substrate. This heat needs to be quickly transferred from the substrate to the chuck and then quickly dissipated from the chuck, so that the substrate is kept at a steady temperature. If the transfer of heat from substrate to chuck, or the heat dissipation from the chuck itself is inefficient or inadequate, the temperature of the substrate increases rapidly. The accumulation of heat in the substrate can damage structures on the substrate (e.g., excess heat can cause unwanted diffusion of dopants in a semiconductor substrate, which can lead to leakage currents in transistors). This thermal buildup also impacts tool throughput because in the absence of an effective heat transfer mechanism, the plasma process needs to be operated at a lower power level or in an interrupted manner (to allow the substrates to cool) to achieve adequate process yield (e.g., fewer damaged devices). Control of substrate heating during plasma processing is also important because the substrate temperature affects the etch process itself (e.g., etch selectivity to photoresist, etc.).
FIG. 1 is a schematic diagram of a prior art chuck apparatus 10 for supporting a substrate W in a low-pressure environment. Chuck 10 includes a chuck body 12 with an upper surface 14 and a conduit 16 formed within the chuck body that leads from a helium gas source (not shown) to surface 14. During substrate processing, helium gas 18 is fed into conduit 16 and flows toward substrate W. Because of the low-pressure environment above the wafer, the helium gas 18 introduced between the wafer and chuck, and the roughness of chuck surface 14, a gas gap forms between chuck surface 14 and the lower surface of substrate W. This gap separates the substrate and the chuck body by a low-pressure gas gap 30 having a mean gap width δ, which is typically a few micrometers wide. In this sense, substrate W is exposed to helium gas 18 flowing between chuck upper surface 14 and substrate W lower surface. The helium gas injected into gap 30 flows outwards to the outer edge of substrate W, its presence in the gap 30 thereby allowing the transfer of heat away from the substrate to the chuck (as indicated by arrows 32). This heat transfer mechanism is known as low-pressure gas gap conduction and is widely used in the semiconductor industry. Helium gas is used to effectuate the heat transfer because it is inert and has a high thermal conductivity (only hydrogen has a higher thermal conductivity).
Unlike the case of heat conduction in normal atmospheric (e.g. high) pressure conditions, under low pressure conditions, the extent of gas-surface energy exchange (and, hence, the effectiveness of cooling) is characterized by the so-called accommodation coefficient α, in addition to the gas thermal conductivity.
Low-pressure (i.e., of the order of 10 to 50 Torr or less) gas gap conduction is used for cooling in many types of substrate processing equipment. For instance, in most etch tools, there is a gap between the substrate and the lower electrode. This gap is filled with low-pressure helium or argon and is used to cool the substrate. To guide the helium or argon flow in a certain direction, various channels on the upper surface of the chuck or on the backside of the substrate may be used.
The heat flux q″ of thermal conduction for a low-pressure gas gap between a substrate and a chuck is given by the product of the heat transfer coefficient hg and the temperature difference ΔT=Tw−Tc between the proximate surfaces of the substrate and the chuck, or q″=hgΔT. In general, the chuck temperature is controlled by its cooling system. The substrate temperature, on the other hand, is constrained by the desire to maintain a high process yield and by the type of devices being fabricated on the substrate. For a given process and device type, this essentially fixes the temperature difference ΔT.